The present invention relates to a method of removing sulfur oxides from a hot gas, and, more particularly, to the removal of sulfur oxides from a hot sulfur-bearing flue gas formed during the combustion of fossil fuels, especially oil and sulfur-bearing coal which are burned at electric power generating plants.
During the combustion of oil and sulfur-bearing coal, various combustion off-gases are produced which contain a variety of contaminants such as sulfur dioxide, sulfur trioxide and flyash. Combustion exhaust gases containing sulfur dioxide present a serious air pollution problem, and, as a result, federal and state air pollution control agencies have promulgated regulations to control the levels of sulfur dioxide released to the atmosphere. Accordingly, many processes have been developed in an effort to meet air pollution emission control standards by effectively controlling the levels of sulfur dioxide emissions generated during fossil fuel combustion.
One approach used for controlling sulfur dioxide from combustion exhaust gases has been to improve the quality of fuel by reducing its sulfur content prior to combustion. Another approach has been to locate and utilize fossil fuels with a low sulfur-bearing content. Still another approach has been to mix a dry alkali compound with a fossil fuel and inject the mixture into the combustion zone.
Combustion exhaust gases have also been treated by a variety of wet absorption processes in an attempt to control sulfur dioxide emissions. Wet absorption processes have been found suitable for treating relatively low temperature flue gases wherein the gas is typically washed with an aqueous alkaline solution or slurry. Wet absorption processes, while useful, have several disadvantages, namely, they have a mist carryover problem which permits moisture to be emitted to the atmosphere in relatively large quantities; they require gas reheat to control atmospheric moisture discharge; they liberate particulate matter contained in the mist during atmospheric mist evaporation; they present a sludge disposal problem in that collected alkali-sulfur reacted materials must either be separated from the liquid in a settling pond or in a de-watering system prior to disposal; and they do not readily lend themselves to regeneration of alkali for use in additional gas treatment.
Many attempts have been made to add alkali in a dry pulverized form directly to combustion exhaust gases to control sulfur dioxide emissions but to date the results have not been found to be commercially acceptable. In such processes, sulfur dioxide is generally removed either by chemical absorption or adsorption followed by oxidation. The disadvantages of adding dry alkali directly to exhaust gases are that reaction time between the alkali and gases is relatively slow and inefficient and reaction by-products are not readily regenerated for the recapture of desirable constituents.
The object of the present invention is to provide an improved method for removing sulfur oxides from a hot gas. The most pertinent existing prior art relating to the removal of sulfur oxides from hot gases is disclosed in U.S. Pat. No. 4,197,278. In this art reference, sulfur oxide control is accomplished by introducing a hot gas in a single compartment spray-drying zone while simultaneously introducing a gas contacting aqueous reactive absorbent and then contacting the gas with the absorbent in a second fabric filter zone. The gas is introduced at the top and removed from the bottom of the spray-drying compartment. The aqueous gas contacting absorbent is introduced and dispersed in the spray-drying zone by directing the absorbent to inpinge upon the surface of a horizontally rotating disc.
The present invention introduces the hot untreated gas into a multiple compartment spray drying reactor chamber. Gas velocity is effectively controlled, settleable entrained particles are removed and thereby not unnecessarily contacted and treated with an atomized alkali slurry and the gas is uniformly introduced and dispersed throughout a spray drying compartment of the reactor chamber. The present invention eliminates the prior art rotating disc and effectively utilizes a series of nozzles which introduce a finely atomized alkali slurry that provides intimate contact with the gas in the spray drying compartment. The costly power source necessary to impart rotation of the prior art disc is thereby eliminated, thus rendering the present system more economical. Additionally, the nozzles provide a more highly dispersed and evenly distributed alkali spray, eliminate the presence of large liquid droplets inherent in the use of a rotating disc, and assure a more intimate gas-alkali contact, thereby providing a more efficient gas treatment proces.